Its semimajor axis is 0.055 AU. This proximity to the G0 star will produce ~450,000 watts/m^2 (comparing it to a 1,367 solar constant value).

I think the models limiting planet size to only slightly larger than Jupiter were not intended to handle hot atmospheres, but this is just an assumption. There may have been no exoplanets when they were developed.

Maybe the planet started off like Jupitor, but it had some chemical process that produced a substance that formed/bubbled/etc at the surface, or condensed out of atmosphere in to particles that floated and attached to other particals and eventually formed a shell surrounding the planet?????
The when the planet cools, the planet shrinks and the shell remains the size it was. Leaving a planet that is basically hollow (except it has a core) and the density being what it is because of this "hollowness"........

The problem is it should never hove gotten that big in the first place. If you add lots of gas to Jupiter, it will increase the mass and, therefore, increase the gravity. This will cause the atmosphere to shrink, somewhat. This minimizes just how big a gas giant can become. They can be larger than Jupiter, but not 30% larger.

However, it wouldn't surprise me if this formulation of maximum size deals with cool and warm planets, not planets getting fried next to their star. A hot atmosphere will expand and I would not expect a 30% increase to cause a concern. I am not an astrophysicist by a long shot, however.